The quest for ever-increasing audio performance has brought about an increased desire to determine the characteristics of a load of an amplifier (i.e., speaker). Detecting the impedance (and any degradation thereof), for example, would enable one to protect the speaker load from being overdriven. The typical approach would be to place components (i.e., a sense resistor) in series with the load. However, the addition of these components degrades the efficiency of the amplifier and creates issues with common mode signals. Thus, it is desirable to perform measurements without the use of external components that would interfere with the normal operation of the amplifier.
Today, class-D amplifiers are desirable because of their high efficiency. Class-D amplifiers employ a pulse width modulator (PWM) that controls the transistors of an H-bridge, which includes high-side and low-side transistors. Because of the structure of the H-bridge and the driving of an inductive load (i.e., speaker), class-D amplifiers and DC-DC converters share some common characteristics. In FIG. 1, an example of a current sensing scheme for a DC-DC converter 100 can be seen. As shown, the switches or transistors Q1 and Q2 (which operate as the high-side and low-side transistors) and the sensing circuit 104 (which generally comprises transistors Q3 and Q4, resistor RSEN, and amplifier 106) are internal to integrated circuit 102, while the inductive load (which generally comprises inductor L, resistor R1, and capacitor C2) are external to IC 102. Here, the load current is replicated across resistor RSEN to generate a sense voltage (which is the voltage drop across resistor RSEN).
The arrangement of circuit 100, however, is inadequate for class-D amplifiers. With DC-DC converters (such as converter 100), voltage swings are relatively small, so one would be able to continuously perform current sense measurements. With class-D amplifiers, on the other hand, the swings are rail-to-rail, which would not allow for continuous current sensing. Therefore, there is a need for a current sensing circuit for class-D amplifiers that would not generally interfere with normal operation of the amplifier.
Some examples of conventional circuits are: U.S. Pat. No. 7,545,207; U.S. Pat. No. 6,600,618; U.S. Pat. No. 6,614,297; U.S. Pat. No. 6,865,417; U.S. Pat. No. 7,194,303; U.S. Pat. No. 7,332,962; U.S. Pat. No. 7,355,473; U.S. Pat. No. 7,388,426; U.S. Pat. No. 7,471,144; U.S. Pat. No. 7,737,776; U.S. Patent Pre-Grant Publ. No. 2002/0141098; and Forghani-zadeh et al., “Current-Sensing Techniques for DC-DC Converters,” Proc. IEEE Midwest Symposium on Circuits and Systems, 2002, MWSCAS, vol. 2, pp. 577-580.